This invention relates to coating using a paint curtain machine and more particularly to depositing uniformly thick dielectric ceramic layers for making capacitors.
Among the methods for forming thin dielectric ceramic layers in the manufacture of ceramic capacitors, the best known are casting, extruding, spraying and curtain coating. It is the improvement of curtain coating for that use that is the subject of this invention.
The thickness of a coating having been so deposited is determined by the thickness of the falling curtain and the speed with which the substrate is passed through the curtain to acquire the paint coating. A curtain coating machine of the kind having a paint reservoir with a dam or wier built into one wall from which the paint falls is described by Coleman in the patent U.S. Pat. No. 4,060,649 issued Nov. 29, 1977 and assigned to the same assignee as is the present invention.
The method described by Coleman entails providing a moving conveyor belt in the curtain. Substrates to be coated are placed on the conveyor belt, passed through the curtain and retrieved on the other side. No means are provided for monitoring the thickness of the falling curtain.
It is known to measure the thickness of lacquer deposited continuously on a translucent web by exposing one side to the light of a lamp and measuring the amount of light transmitted through the combination web-with-coating as a measure of coating thickness. However, any changes or non linearities in the thickness and light transparency of the translucent web become a source of uncertainty and error in that coating thickness measure. Furthermore, for making ceramic capacitors it is desirable to put down successive coatings of a ceramic paint interleaved with buried electrodes wherein control of thickness of each layer is needed, in which case the prior art method becomes ineffectual altogether for measuring the successive layers.
It is therefore an object of this invention to overcome the above-noted limitations of the prior art.
It is an object of this invention to provide a method for obtaining a measure of the curtain thickness.
It is another object of this invention to provide such a method wherein both sides of the falling curtain are monitored and the reservoir is physically adjusted for providing a uniformly thick curtain and thus providing a uniformly thick paint coating everywhere on the substrate.